Pale like when beheaded (5)
I believe the answer is:
ashen
'pale' is the definition.
('ashen' can be a synonym of 'pale')
'like when beheaded' is the wordplay.
'like' becomes 'as'.
'beheaded' means to remove the first letter.
'when' with its initial letter taken away is 'hen'.
'as'+'hen'='ASHEN'
(Other definitions for ashen that I've seen before include "of Tree" , "showing signs of anaemia" , "lacking in colour" , "Anaemic-looking" , "Anaemic looking" .)